Guardrails are traffic barriers placed along roadsides to screen arrant vehicles from hazards behind the barrier. A common guardrail in the U.S. is constructed using a standard steel W-beam mounted on spaced wood or steel posts. Because the W-beam functions primarily in tension when redirecting impacting vehicles, a function of the end is to provide necessary anchorage for the beam to develop necessary tensile forces. In addition, since the guardrail end represents a discontinuity in the barrier system, it is subject to being struck “head-on” by vehicles with small departure angles from the roadway. When struck in this manner, the end might spear the vehicle. Some widely used terminal designs “bury” the W-beam at the end to eliminate spearing, but this design may have shortcomings, including causing problems related to vaulting and rollover due to the vehicle riding up the end, and subsequently becoming airborne.
Another type of highway safety device are cable safety systems and cable barriers, which have been installed along edges of roadways and highways for many years. Cable safety systems and cable barriers have also been installed along medians between roadways and/or highways. Cable safety systems generally include one or more horizontal cables attached to support posts. For some applications cable safety systems and cable barriers may reduce damage to an impacting vehicle and/or injury to occupants of the impacting vehicle as compared with other types of highway safety systems and highway barriers.
Cable safety systems are often designed and installed with at least one cable mounted horizontally on a plurality of generally vertical support posts. The number of cables may vary depending on various factors such as the type of vehicles using the associated roadway and the hazard which required installation of the cable safety system. The length of a cable safety system is generally determined based on the adjacent roadside hazard. Each cable is typically installed at a selected height relative to the ground and with selected vertical spacing between adjacent cables. Associated support posts are installed with desired horizontal spacing between adjacent posts.
One recognized limitation of many cable safety systems is excessive deflection of associated cables during a vehicle impact. Deflection associated with a cable safety system may be larger than deflection of a convention W-beam guardrail when subjected to the same type of vehicle impact. Such deflection frequently determines maximum allowed spacing between adjacent posts for satisfactory performance of the cable safety system. Large deflection during a vehicle impact also increases the risk of the vehicle running over the cables and being exposed to the hazard, which required installation of the cable safety system. Calculating performance of many cable safety systems is often difficult due to unpredictable interactions between associated posts and cables during a vehicle impact. Depending upon car type, speed and angle of impact, cables may release as far as ten (10) or most posts spaced ahead of the impact location. Cable release from posts often causes much larger deflections than expected or calculated.
From full scale crash testing and from real life experience, it has been determined that keeping the length of unsupported cables as short as possible will generally reduce deflection. The longer the distance between adjacent posts supporting associated cables, the larger the deflection will generally be during a vehicle impact. An increased number of posts (shorter post spacing) will generally decrease deflection. However, shorter spacing between posts affects total cost of a cable safety system, not only material, but also installation cost.
During the past several years, cable safety systems have been used as an alternative to traditional W-beam guardrail systems. These cable safety systems address some of the weaknesses of prior cable safety systems by using pre-stretched cables and/or reducing spacing between adjacent posts to reduce deflection to an acceptable level. A consultant report “Dynamic Analysis of Cable Guardrail” issued in April 1994 by an ES-Consult in Denmark, established a model for various parameters, which affect performance and design considerations for acceptable deflection of cable safety systems.
Standards have been developed within the European standardization body, CEN (Commite Europeen de Normalisation), for impact tests performed on safety systems and barriers. These barrier impact tests are described in CEN 1317, Road Restraint Systems. According to the CEN standards, safety systems and barriers are to be impact tested at different containment levels. The elongation or deformation of a barrier is also measured to determine a safe working width. The environment in which the barrier is to be constructed generally determines appropriate containment level as well as permissible working width. The CEN standard generally requires that the risk of injury in a collision with the barrier is minimized (injury risk class). CEN standards are used in the European countries and several countries near Europe, among others.
NCHRP stands for the National Cooperative Highway Research Program, a program developed by the Transportation Research Board of the National Research Council, USA. Report 350 is entitled “Recommended Procedures for the Safety Performance Evaluation of Highway Features” and may be commonly referred to as the NCHRP 350 Standard. The standard describes how impact tests should be conducted. Test results may be used to determine elongation or deformation and safe working widths. This standard is used mainly in the USA and is predominately used in Australia and New Zealand. SUMMARY OF THE INVENTION
In accordance with teachings of the present invention, a combined guardrail and cable safety system is provided which overcomes many disadvantages and problems associated with prior guardrail safety systems, cable safety systems and cable barriers.
A safety barrier incorporating teachings of the present invention may include a plurality of cable posts spaced from each other and disposed adjacent to a roadway. At least two cables may be releasably engaged with and supported by the cable posts. The cable posts and the at least two cables cooperate with each other to prevent a vehicle from leaving the roadway. A plurality of guardrail posts may also be spaced from each other and disposed adjacent to the roadway longitudinally spaced from the plurality of cable post. A guardrail beam may be coupled to the plurality of guardrail posts. The at least two cables extend from the cable posts to engage respective cable anchor brackets attached to portions of the guardrail beam.
For some embodiments, a combined guardrail and cable safety system may include a guardrail safety system extending along a roadway. The guardrail safety system may have a plurality of guardrail posts. Each guardrail post may be operably coupled to a longitudinally corrugated guardrail beam having at least two crowns and a valley disposed between the crowns. One or more cables may extend from a cable safety system such that the one or more cable may be received at the guardrail beam and traverses within the valley of the guardrail beam. An elongated slot may be disposed in and extend longitudinally along the guardrail beam. The slot may be sized to receive a portion of the one or more cables therethrough. A cable anchor bracket may be fastened to a portion of the guardrail safety system. The cable anchor bracket may be operable to receive and secure the portion of each of the one or more cables protruding through the slot such that sufficient tension is applied to the respective cable.
In further embodiments, a guardrail beam having a cable anchor assembly for joining a guardrail safety system to a cable safety system may include a longitudinally corrugated rail having a W beam configuration defined in part by a valley disposed between two crowns. The beam may be operably coupled to a plurality of support posts that extend along a roadway. The valley includes an elongated opening extending longitudinally along the valley such that the opening receives a cable extending from the cable safety system formed adjacent the guardrail safety system along the roadway wherein one end of the guardrail system is contiguous with one end of the cable system. A cable anchor assembly having a cable anchor bracket secured to a portion of the beam adjacent to the opening. The cable anchor bracket operable to receive an end portion of the cable such that the cable may be secured to the beam.
Technical benefits of the present invention include providing a combined guardrail and cable safety system that maintains adequate barrier protection during a transition between cable and guardrail safety systems. Because the cables may be coupled directly to associated guardrail beams, the cable will have adequate tension adjacent to the junction between the cable system and the guardrail system. Additionally, attaching the cables directly to the beams will transfer forces from an impacting vehicle to both the cable safety system and the guardrail safety system.
Additional technical benefits of the present invention include a combined guardrail and cable safety system that has cables with less tension and greater spacing between associated support posts. Due to the transition between the cable safety system and the guardrail safety system, the combined guardrail and cable safety system maintains satisfactory deflecting characteristics.
Further technical benefits of the present invention include an economical design and the employment of component parts. Repairs or replacement of damaged components may often be more easily accomplished after a vehicle impact due to each component's design. The need for periodic re-tensioning of cables may be reduced or eliminated by the present invention.
Further technical benefits of the present invention include a flexible transition junction between a guardrail safety system and cable safety system. Because cables extending from the cable safety system may be attached with associated guardrail beams, a smoother transition is possible from the cable the system to the guardrail safety system. Thus, designers have more flexibility in design and installation of cable and/or guardrail safety systems as determined by specific highway and roadway conditions.
The present invention allows differences in design and performance of cable and guardrail safety systems to be combined into an appropriate safety system for a wide variety of highways and roadways. The present invention provides a smooth transition between a cable safety system and a guardrail safety system. The present invention provides a combination of guardrail and cable safety system designs which assists in joining or bridging the two safety systems.
All, some, or none of these technical advantages may be present in various embodiments of the present invention. Other technical advantages will be apparent to one skilled in the art from the following figures, descriptions, and claims.